1. Field of the Invention
This invention relates to machines for compacting and forming bales of material in a continuous extrusion type baling machine, and in particular to tensioning means for holding the previously formed bale in the throat portion of the machine.
2. Description of the Prior Art
Baling machines for baling waste paper and other industrial refuse are known and are generally used for the purpose of forming the material into bales which can be easily handled while they are being transported to a place where the material is to be reprocessed. Such baling machines conventionally comprise a horizontal baling chamber having sides for constraining the bale laterally and an open outlet end of controllable cross section from which the bales are extruded, and a feed hopper inlet opening into the top side of the baling chamber for delivering material to form the bale. A baling head reciprocates in the baling chamber past the inlet opening between a rearward position of retraction and an advanced position of extension. The baling head is conventionally driven by a fluid cylinder, specifically a hydraulic cylinder. In operation, a charge of compressable material is dumped into the hopper and drops into the baling chamber while the baling head is retracted. Successive charges are compressed and compacted together in the baling chamber against the resistance of the material previously compressed and being extruded through the throat by repeated strokes of the baling head. In this manner a length of compacted and compressed material is formed and extruded through and out the outlet open end of the baling chamber.
The baling chamber is typically provided with an orifice portion of minimum cross-sectional area or a throat which can be varied by a tension mechanism driven by fluid or hydraulic power derived from the same hydraulic system powering the compression and extrusion baling head. The throat size varies the resistance to the outflow of compressed material from and through the baling chamber. As this tension or resistance to outflow increases, the hydraulic pressure in the cylinder driving the baling head must increase. If the resistance becomes too great, the machine stalls. The intended operation is interrupted, excessive heat is developed, and power is wasted.
Such a machine, together with the material to be baled passing through the machine, thus represents a hydraulic-mechanical closed loop system. The hydraulic cylinder operating the baling head and the actuated tension mechanism controlling the orifice and resistance to the flow through the baling chamber are interconnected on the one hand through the main hydraulic system and on the other hand through a mechanical path comprising the baling head, the movable orifice-defining walls of the baling chamber and the compressed material between them.
It is desirable and efficient to operate such machines near the maximum pressure that can be safely developed by the hydraulic system. Maximum pressure produces compressed or extruded bales of the greatest possible density. Efficient operation also requires that the machine not be operated into a stalled condition.
One way to prevent stalling is to use pressure relief valves in the main hydraulic system. Such relief valves are effective to prevent stalling but greatly reduce the efficiency of the machine's operation. For example, the pressure rise in the hydraulic cylinder operating the baling head is generally hyperbolic. The very sudden and steep rise in pressure experienced just prior to stalling requires main-system type relief valves to be set well below maximum deliverable system pressure if the system is to be adequately protected. This represents wasted pump capacity. Additionally, such relief valves act to dump main system pressure which must be re-established before operation can be resumed. Time and power are wasted. Because of the excessive pressure developed in the hydraulic cylinder operating the baling head is not dealt with and still must, of course, be removed or the load relieved in some manner before beginnning operation again.
Another way to prevent stalling is to dump the main system pressure and the pressure from the cylinder operating orifice-controlling tension mechanism of the machine. This is typically accomplished by an unloading valve inserted in a line branching off to a tank from a line connecting the hydraulic tension cylinder to the main system. This system also suffers from the disadvantage of permitting main system pressure and fluid to be lost to the pressure side of the system. Also, to the extent that the main system fluid is lost through such an unloading valve, fluid from the tension cylinder is prevented from being unloaded and the problem and its solution is thereby prolonged at the expense of efficiency.
U.S. Pat. No. 3,467,000 describes a control system for a hydraulically-operated orifice-controlling tension cylinder which includes a pilot-operated check valve connected to the cylinder. The flow of fluid to the tension cylinder through the check valve is controlled by a second valve which is movable between a first position in which supply pressure is supplied to the tension cylinder through the check valve and a second position in which supply pressure is supplied to the pilot pressure port of the check valve to cut off the flow of fluid to the tension cylinder. The second valve is operated by a second pilot line connected to the supply pressure so that the valve changes position when the pressure supply in the conduit exceeds a predetermined level. This system prevents stalling of the machine by timely relieving the excessive resistance load responsible for the impending stall. It also restores the machine to its normal operating condition after relieving the excessive load.
The control system described in the aforementioned U.S. Pat. No. 3,467,000 however, requires the use of a specialized relief valve which is relatively expensive to fabricate. In addition, this control system will only accept a supply pressure up to a set pressure of about 1,000 psi. The control system must include means for reducing the supply pressure above this set pressure to assure that the pressure supplied to the tension cylinder is at or below 1,000 psi. Up to 1,800 psi, the supply pressure is received and reduced by this pressure reducing means. In the event that the supply pressure exceeds 1,800 psi, the control system must include means for fully relieving all pressure. This pressure relief means dumps the system pressure to zero and opens the tension cylinder to relieve pressure on the bale on the throat portion. Thus the control system includes many of the disadvantages of the aforementioned control systems in that system pressures may be undesirably dumped.
Other control systems for maintaining proper tension on the extrusion throat of the baling machine are shown in U.S. Pat. Nos. 3,350,999, issued to Morse; 4,059,049, issued to Tillgren; 4,168,659, issued to Yatcilla et al.